1. Field of the Invention
The present invention relates to porous metallic material. More particularly, the present invention relates to a method for the preparation of open cell porous metallic material, which is applicable to filters, electrodes for fuel cells and the like, and other suitable uses.
2. Description of the Prior Art
Several open cell porous materials including those of metals and of ceramics are used to filter various gases and solutions of agents during the production of semiconductors. In particular, the former finds its use in electrodes for cells, alloys for hydrogen storage, and others. The present invention is directed specifically to open cell porous metallic material.
It is difficult to define the requirement for open cell porous metallic material in general, because of its dependence on the use thereof. In the use, however, to which the present invention is intended to be applied, and in which fine particle flow is involved, the requirement includes existence of fine and uniformly distributed micropores, mechanical stability of the material, large pore volume or porosity, etc.
In the prior art, methods have been proposed to prepare open cell porous metallic material, wherein the raw material is provided from a certain metal powder of uniform particle size, or fibers, a binder is then added thereto, and after compression molding, the mixture is thermally treated in a non-oxidative atmosphere at an appropriate temperature to be sintered in part [see Yamagata Prefectural Industrial Technology Center Report, No. 21 (in Japanese); Mizuki et al., Kogyo Zairyo, 30(10), 89 -99 (1982)]. Preparing a metal powder of small particle size, however, is carried out using such method as spraying melted metal, or cutting wire rods and subsequent milling [see e.g. Kinzoku Binran, "Preparation of Powders" Sect.; Japanese Patent Application Kokai Nos. 55-93,701, 56-12,559 and 56-52,146], making the powder expensive. Moreover, because of the large surface area and high risk of ignition entailed in such powders, operation in air such as during molding etc. is difficult. Consequently, there arise problems that utmost care is required in the preparation, and that the cost reaches a large amount. Using powders of larger particle size will result in failure to realize sufficiently fine micropores.
In terms of open cell porous ceramic material, there exist several disadvantages, including a possibility of shedding (peeling off of the material from the surface), and an inability of welding to metals for mounting to their supports. Also the material involves a problem of lower porosity, which plays an important role in the application to filters.
Further, problems also reside in porous polymer membranes, which, while being used widely, typically are of low thermal resistance, of insufficient strength, and unable to weld to metals.
While open cell porous metallic material in the prior art has such disadvantages as stated above, it has several advantages in that it is free from the possibility of shedding, and easily weldable to metals, as compared with porous ceramic material on the one hand, and highly thermally resistant, promising sufficient strength, and again easily weldable to metals, as compared with porous polymers on the other hand. Thus, we have concentrated our study to open cell porous metallic material to have finally contrived a readily practicable method for its preparation in a stable state, as compared with those methods in the prior art.